Purpose: We evaluated the preoperative clinical factors that affect the surgical outcome of posterior urethral anastomosis (PUA) with a gracilis muscle flap (GMF) to determine which factors predict benefit from the use of the GMF. Materials and Methods: This was a retrospective analysis of 49 patients who underwent a delayed PUA with a GMF. A successful clinical outcome was defined as achieving a peak urinary flow rate greater than 15 mL/s at 3 and 12 months postoperatively without evidence of stricture recurrence on a retrograde urethrogram or cystourethroscopy at 3 months postoperatively. Multiple clinical factors were evaluated by use of univariate and multivariate analyses. Results: The outcome of 21 of 49 patients (42.9%) was deemed successful. The mean age of the 49 patients was 37.2±13.5 years and the mean follow-up duration was 43.4±28.0 months. The length of the urethral defect was significantly shorter in patients with a successful outcome than in patients with an unsuccessful outcome (p=0.010). The outcome differed significantly depending on whether the patients had a previously successful urethroplasty (p=0.036) or whether they had suffered a pelvic bone injury (p=0.012). Multivariate logistic regression analyses revealed that a previous urethroplasty was the only preoperative clinical factor that significantly affected the surgical outcome in PUA with a GMF (odds ratio, 0.218; 95% confidence interval, 0.050 to 0.947; p=0.042). Conclusions: A history of previous urethroplasty is a preoperative clinical factor that significantly affects the surgical outcome in PUA with a GMF; the procedure is more likely to be successful in patients who have not previously undergone urethroplasty.